A typical pulse combustor (or pulsejet engine) consists of a combustion chamber, an inlet pipe, a fuel injector, a spark plug (or other ignition means), and an exhaust pipe. The combustion chamber, inlet pipe and exhaust pipe are often cylindrical, but are not limited to such geometry and can take on a variety of shapes. The diameter of the inlet and exhaust pipes is generally smaller than the diameter of the combustion chamber, and the length of the inlet pipe is generally significantly smaller than the length of the exhaust pipe.
The advantages of pulse combustors include their ability draw in fresh air and sustain operation without any external machinery or moving parts. Pulse combustors can also be used as thrust-producing devices, in which case they are commonly referred to as “pulsejet” or “pulse jet” engines. Pulsejet engines have been in use for a long time and have been used to propel several types of aircraft over the last century. Pulsejet engines are often characterized by a diverging exhaust pipe to aid in thrust production.
Pulsejet engines are characterized by their simplicity, particularly because of the lack of moving parts. However, the oscillating nature of the flows into and out of the pulsejet engines tends to produce very high noise and vibration levels that have often been cited as the most serious hurdles in the widespread implementation of pulsejet engines. One particular goal in developing improved pulsejet engines is addressing the high noise and vibration levels. Further, another goal in developing improved pulsejet engines is improving efficiency, mechanical energy conversion, and/or thrust from pulsejet engines.
A proposed application for pulsejet engines involves aircraft with Vertical Take-Off and Landing (VTOL) capability, for example, such as that proposed in U.S. Pat. No. 6,793,174 B2. In such an aircraft, an array of pulsejet engines mounted under/inside the fuselage/airframe provide vertical lift for takeoff and landing. However, arrays of pulsejet engines produce high noise and vibration levels, which prevents their widespread implementation. A further goal in developing improved pulsejet engines is addressing the high noise and vibration levels produced by two or more pulsejet engines.
It has previously been proposed that one way to counter the oscillating nature of a pulsejet engine is to operate two pulsejet engines simultaneously but in anti-phase. In this manner, the oscillating nature of one pulsejet engine is countered by the other. An arrangement to produce such operation between two pulsejet engines has been designed and tested by several researchers, for example, in U.S. Pat. No. 4,840,558 A. This arrangement requires the exhaust pipes of two pulsejet engines to be connected via a chamber with relatively large volume and/or requires the inlet pipes of two pulsejet engines to be connected via a chamber with a relatively large volume. These connecting chambers are often referred to as ‘decoupling chambers’. While this arrangement has been successful in producing anti-phase operation, it also has the detrimental effect of reducing oscillation pressure amplitude of the engines, as has been noted by several researchers, including, for example, R. G. Evans and A. S. Alshami in their paper Pulse Jet Orchard Heater System Development: Part I. Design, Construction, and Optimization, the disclosure of which is incorporated herein by reference in its entirety. This loss of oscillation pressure amplitude manifests itself as a reduction in useful mechanical power output, or in the case of a thrust-producing pulsejet, as a loss of thrust. The loss of pressure amplitude can occur for several reasons, one of which is that the insertion of a decoupling chamber between the intake pipe and the atmosphere provides higher resistance to flow drawn into the combustion chamber from the atmosphere, as compared to the case with no decoupling chamber. The result is that a smaller amount of air enters the chamber for a given pressure difference between the combustion chamber and the atmosphere, resulting in lower amounts of fuel that can be burned, and subsequently, lower energy release per cycle. Similarly, the insertion of a decoupling chamber between the exhaust pipes of pulsejets impedes the high-velocity exhaust gases, also leading to thrust loss.
None of the prior tools are especially well optimized for operating two pulsejet engines (or pulse combustors) in anti-phase. One goal in developing improved systems and methods for operation of pulsejet engines (or pulse combustors) is to provide an arrangement which would produce anti-phase operation between two pulsejet engines (or pulse combustors) with minimal interference in the operation of the individual pulsejet engines (or pulse combustors).